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Full text of "Handbook Of Chemical Engineering - I"

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PYROMETRY                                         433
of very low line resistance is not of great importance, but the wiring should be welHnstalled for the psychological effect if nothing else. Indicating and recording instruments usually should be mounted upon switchboards, except of course in the case of portable indicators, with suitable selective or commutating switches when several couples are used with one indicator.
When the head of the couple is exposed to severe conditions, rain, etc., as in out side kilns, a weather-proof terminal head should be used. This consists of an outside casing which fits over both binding posts. The cover may be tapped for conduit wiring or provided with a packing gland or stuffing box if a length of flexible cable is used between the couple and the conduit. The lead wires should be carried from the couple to the indicator through rooms as cool as conveniently possible. Copper has a high temperature .coefficient of resistance and the frequent practice of running wires over the top of a long row of furnaces may cause large variations in line resistance.
The indicator or recorder should be conveniently located. If the instrument is desired especially for the use of the operator of a furnace, it should be placed where it is readily available. It should be mounted where vibration and excessive dirt and dust will not injure the delicate parts of the mechanism. In almost all industrial installations outside protecting cases are required to prevent dust from filtering through the case of the indicator. The various screws and gears of a recorder require occasional attention. The tools for this work, screw-drivers, oil can, etc., may be mounted inside the outer protecting case where they are available for immediate use. Special devices are employed to damp out vibration when this is serious, as in the neighborhood of a trip hammer or rolling mill. Frequently the instruments are suspended on spiral springs. One convenient method especially suitable for heavy instruments such as a recorder, is to mount the instrument on a shelf which is supported on a pier by four ordinary tennis balls, one at each corner of the shelf. The tennis balls damp out vibration very satisfactorily.
The proper location of couples in the furnace depends upon the particular process and use to which the furnace is put. The primary consideration must be to locate the hot junction at the point the temperature of which is required. A secondary consideration, however, is to locate the couple where the lead wires may be conveniently taken care of. The space between the protecting tube of the couple and the furnace wall should be tightly plugged with refractory cement, so that hot air cannot strike through the hole on to the head of the couple, nor cold air be drawn in thus cooling the hot junction of the couple.
The use of extension or compensating lead wires, cold-junction boxes, etc., has been discussed elsewhere. The cold-junction box should be so located as to reduce t-he amount of compensating lead wire required to a minimum. Compensating lead wires arc somewhat costly and should not be employed extravagantly. Also the use of long lengths of compensating wire increases the line resistance since the resistivity of these leads is much higher than that of copper.
In case the cold junction is buried under ground, it must not be located too near a large furnace. Either the distance from the furnace, or the depth at which the junction is buried must be increased. A depth of 10 ft. at at least 10 ft. from a large furnace is usually satisfactory.
The use of a common return wire for a multiple installation is in general unsatisfactory. With such installations, short-circuits through the metal protecting tube of the couple to the furnace and other couples are likely to occur. The trouble which the short-circuits can produce is sufficient to warrant the extra cost of copper required to prevent them. With the common return, leakage from a power installation affects the reading of every couple connected to the return and a leakage through a high resistance may alter the readings of every couple by the same amount so that